The origin(s), differentiation and renewal of secretory (mucous) cells during regeneration following traumatic injury, and the lability of secretory cells and the origin(s) of goblet cells which arise after intratracheal instillation of elastase in saline will be characterized in hamster tracheal epithelium. Morphological (high resolution light microscopy and electron microscopy), autoradiographical and histochemical techniques will be used to quantitate the respective roles played by basal cells and secretory cells during epithelial regeneration. Secretory cells, although differentiated, divide to produce daughter cells during the regenerative process similar to the situation in liver and kidney where differentiated parenchymal cells divide when required for regeneration following lethal cell injury. The genealogy of all cells which arise during regeneration will be characterized and quantitated. Proof that undifferentiated "indifferent cells" arise in the epithelium prior to reestablishment of normal architecture, will be sought.